A thermal therapy has been used for treating various diseases. For example, an interventional treatment such as a minimally-invasive treatment of cardiac arrhythmias (e.g. an atrial fibrillation (AF)) and other diseases is possible. In such treatment, cardiac tissue is denaturised by a thermal therapy. Heated muscle cells in cardiac tissue are denaturated and lose their biological function, which can be measured by an increase in the tissue impedance. Furthermore, in oncology, cancer cells are heated up in order to destroy their biological function. Different kinds of tissue and other material can also be thermally treated, not only for therapy applications but also for other purposes.
In the minimally-invasive treatment of cardiac arrhythmias and other diseases, ablation catheters are the most commonly-used therapy tools. However, catheter ablation procedures still have significant drawbacks, and research and development continue in this active field. One major drawback is in controlling the ablation settings during treatment. Currently, the therapists rely on their own expertise to guess the optimal parameters for ablation, such as power, temperature, and duration. These settings vary largely, due to sizable intra-patient differences of e.g. thickness of the local heart wall, perfusion, blood pressure and velocity, heart rhythm, etc.
A thermal therapy is mainly performed by using radio frequency (RF) catheters (in heart tissue ablation), but laser light and high intensity focused ultrasound (HIFU) are used as alternative energy sources. The main benefit of using laser light is a high level of miniaturization, since laser energy can be transported through a very thin fiber. Furthermore, such intervention can be performed without any adaptations in combination with magnetic resonance imaging (MRI), since fibers are MR safe and compatible.
However, when a laser source is used for a thermal therapy, an overheating may occur and there are limited possibilities for control of the thermal process. A major risk related to a catheter therapy is attributed to the overheating of the ablation site. In the case of overheating, either rupturing of the tissue at the ablation or treatment site (releasing potentially life-threatening particles into the blood stream) or damage to neighboring organs and tissues is inflicted. In the case that other organs are affected, fistulas can develop. Such fistulas are often life-threatening. For example, a fistula in the esophagus has a mortality rate of roughly 75%.
An overheating occurs at a point where laser energy is deposited into the tissue. In order to prevent such overheating, an irrigation can be performed, or the temperature of the tissue can be measured. However, if an irrigation is performed, at least one irrigation tube needs to be added to the fiber transporting the laser energy. Furthermore, if an external irrigation is performed, then, during the thermal therapy, additional liquid is pumped into a treated patient, which limits the time of the procedure, but can also cause some complications and side effects. On the other hand, the temperature cannot be measured at the point where an overheating first occurs. If any sensor is placed at that point, then laser light directly heats up the sensor. Therefore, the sensor mainly measures temperature due to light absorption of the sensor, while it should measure the temperature of the heated tissue.